With oil due to run out soon, the hunt is on to find alternatives to petrol. So what will we be filling our cars with in future?

Already there has been a lot of interest in biofuels - combustible liquid fuels derived from replenishable vegetable matter or crops, such as soy or rapeseed. But while these have been touted as having zero emissions - producing no more greenhouse gases than are soaked up by the crops - they are likely to prove far less sustainable than they might at first seem.

For biofuel requires a huge amount of energy to grow and harvest the crops, and refine and transport the fuel. What's more, there are growing concerns that land used to grow biofuel is encroaching on land needed to grow food. And then there's the chemical pollution produced by pesticides and herbicides, not to mention the crops' additional drain on precious water resources.

So what about electric vehicles? Hybrid vehicles are already common. They are usually petrol-powered cars that use electric motors to boost their main engine, so less fuel is consumed. But fully electric cars could soon be available, too. One Australian development, called Trev, is capable of doing 0-60mph in 10 seconds, with a top speed of 74mph.

All very impressive, but the reality of electric vehicles is that they still lack the kind of oomph we're used to getting from petrol engines. This is largely due to the fact that most batteries are unable to deliver power quickly enough, and those batteries that can generally have a tendency to explode with impact - not ideal for automobile applications.

A somewhat retro solution is steam. It may sound odd, but steam engines have the potential to run more efficiently than traditional combustion engines because they can transfer the energy generated more directly to the wheels, so they require no gears. A German company called Enginion has been developing such a car. But there is a catch: although running cleaner than a combustion engine, even these steam engines need fossil fuels to generate the steam.

A French company called MDI Enterprises aims get round this problem by using the next best thing to steam: compressed gas. The company claims it will have a car available on the market within the year that can be refuelled by a compressor in just three minutes, and will be capable of a top speed of nearly 70mph.

Perhaps the most feasible alternative to petrol is the development of hydrogen fuel cell cars. Fuel cells are like batteries that can be repeatedly topped up with liquid hydrogen. The cell mixes the hydrogen with oxygen from the air to produce electricity and steam. Fuels don't come much cleaner than that, so it's hardly surprising that companies such as BMW and Mercedes are now developing hydrogen cars.

But although hydrogen has a higher energy density than petrol in terms of mass, its actual density is very low, making it difficult to store and transport. In fact, with a density 10 times less than water, hydrogen fuel tanks would have to be huge if the vehicles were to have the same range as existing cars. In light of this, the US department of energy has put a bounty on any material that can be developed that is capable of chemically storing hydrogen efficiently.

But nothing will be achieved if the energy it takes to produce these fuels comes from burning fossil fuels elsewhere. If cars are to have a future their fuel must ultimately depend upon a renewable energy source.